1. Field of the Invention
This invention relates to a film forming method in which a substrate is introduced into a reaction chamber having a gas feed port and a gas exhaust port, subjected to predetermined processing and taken out of the reaction chamber. More specifically, it relates to a film forming method in which nonuniformity or irregularities in a film formed on the substrate can be prevented by precluding film-forming gas components, which have been attached to the gas exhaust port and then evaporated therefrom as a film-forming gas, from flowing back to the reaction chamber.
Throughout the description which follows, the term "an exhaust port" refers to an exhaust port and its vicinity in which the exhaust port is connected with a reaction chamber or tube.
2. Description of the Prior Art
When thin layers or films are to be formed on a substrate, a film forming apparatus of a sheet-fed type has been used for example. The term "sheet-fed type", used broadly herein, means that one or more sheets of substrates are simultaneously processed in a successive manner. As a concrete example of such a film forming apparatus, a description will be made of the formation of tantalum oxide (Ta.sub.2 O.sub.5) films on a substrate. Generally, tantalum oxide films are formed by use of a chemical vapor deposition (CVD) process.
FIG. 5 is a schematic view showing an example of a conventional tantalum oxide film producing apparatus. Penta-ethoxy-tantalum in a liquid state is used as a raw material for tantalum oxide films. The penta-ethoxy-tantalum liquid is received in a tank 41 which is located in a thermostatic chamber 42. The temperature of the tank 41 is controlled to a constant value such as, for example, 35 degrees C by means of the thermostatic chamber 42. The interior of the tank 41 is pressurized by a nitrogen (N.sub.2) gas supplied thereto through a nitrogen feed pipe 48 to push out the penta-ethoxy-tantalum liquid into a material feed pipe 49. The penta-ethoxy-tantalum liquid is then supplied from the material feed pipe 49 to a carburetor 43, into which a nitrogen carrier gas is supplied from the nitrogen feed pipe 48. The film-forming gas evaporated by the carburetor 43 is introduced, together with the nitrogen carrier gas, into a reaction chamber 45 through a feed pipe 44. Simultaneously, an oxygen gas is also introduced from an oxygen tank (not shown) into the reaction chamber 45, in which the penta-ethoxy-tantalum liquid is thermally decomposed to form a tantalum oxide film on the substrate. After the film formation, the atmosphere or gases in the reaction chamber 45 is exhausted by means of a discharge pump 46 through an exhaust pipe 47.
In the prior art technology described above, in order to provide a uniform formation of a tantalum oxide film on a substrate, certain proposals have been made for the configuration of the reaction chamber 45, an introduction recipe of the film-forming gas, an exhaust recipe thereof, etc.
For example, Japanese Patent Application Laid-Open No. Hei 7-94419 discloses a semiconductor processing apparatus in which a flat reaction tube is disposed in a heating space defined by a pair of parallel plate heaters, and a substrate to be processed is introduced into the flat reaction tube and subjected to a film forming processing therein. In this semiconductor processing apparatus, the flat reaction tube is provided at its opposite ends with gas feed ports and exhaust ports, so that during the film forming processing, the direction of flow of a reaction gas, which is supplied from the gas feed ports to the reaction tube and exhausted therefrom through the exhaust ports, can be changed arbitrarily.
FIG. 6 illustrates a reaction chamber or tube 51 and its related portions of the semiconductor processing apparatus as disclosed in the above reference. In this figure, an unillustrated substrate is horizontally disposed substantially in the center of the interior of the reaction tube 51, and gas feed ports 52, 53 and gas exhaust ports 54, 55 are provided at opposite ends of the reaction tube 51, the gas feed ports 52, 53 being opposed with respect to the gas exhaust ports 54, 55, respectively, with the substrate being interposed therebetween. For example, a gas supplied from the gas feed port 52 passes through the reaction tube 51 substantially in parallel with the substrate to be exhausted from the gas exhaust port 55, as indicated by an arrow in FIG. 6. At this time, the gas feed port 53 and the gas exhaust port 54 are both closed by unillustrated valves, respectively, to interrupt the passage of the gas. With this conventional apparatus, the direction of the gas flow can be set reversely so that a gas is supplied from the gas feed port 53 to the reaction tube 51 and exhausted from the gas exhaust port 54 while closing the gas feed port 52 and the gas exhaust port 55.
A conventional film-forming recipe for forming a tantalum oxide film on a substrate by use of the semiconductor processing apparatus as disclosed in the above-mentioned Japanese Patent Laid-Open No. Hei-94419 will now be described while referring to the accompanying drawings.
FIGS. 7(a) through 7(c) illustrate the various states of ventilation or gas flows in the reaction tube 51 from a stand-by state to the end of a substrate heating step. Here, note that the substrate heating step is to heat, prior to the formation of a film thereon, the substrate to a desired temperature by a heater (not shown) and to being a surface (i.e., film-forming surface) of the substrate into a uniform state. Preferably, the heater is an electric resistance heater, and it is preferred to employ a hot-wall type heating system in which the temperature of the reaction chamber is held at the desired temperature before the introduction of the substrate into the reaction chamber. The heater may, of course, be a lamp, a high frequency heater, and the like.
In these figures, note that the opening state and the closing state of each gas feed port and each gas exhaust port are indicated by a white circle (valve opening) and a black circle (valve closing), respectively; that the presence of two of white circles and/or black circles indicates the degree or extent of opening or closing of these ports; and that arrows with no symbols designate gas flows. Also, one of the gas feed ports and the gas exhaust ports provided at one end (e.g., at the left side of FIGS. 7(a) through 7(c)) of the reaction tube 51 is designated by the term "back-side", and the other of the gas feed ports and the gas exhaust ports provided at the other end (e.g., at the right side of FIGS. 7(a) through 7(c)) of the reaction tube 51 is designated by the term "front-side".
FIG. 7(a) shows the flow of a gas in the apparatus of the stand-by state. In this stand-by state, valves 61 through 64 respectively opening and closing the ports 52 through 55 (see FIG. 6) are adjusted such that a nitrogen gas flows in a direction from the back-side feed port to the back-side exhaust port and further from front-side feed port to the front-side exhaust port. The gas passing the reaction tube 51 is discharged to the outside by means of a discharge pump (DP) through the exhaust pipe 47. Here, note that the stand-by state means a state prior to the substrate introducing step in which a substrate is introduced into the reaction tube 51. Also, though not illustrated, during the substrate introducing step, all the gas feed ports are closed by the corresponding valves 61, 62 and all the gas exhaust ports are opened by the corresponding valves 63, 64 so that the reaction tube 51 is exhausted or vacuum drawn by the discharge pump (DP) from the exhaust ports via the exhaust pipe 47 so as to keep the interior of the reaction tube 51 at a desired pressure.
FIG. 7(b) shows the flow of a gas in the apparatus during the substrate heating step. In the substrate heating step, a nitrogen gas supplied from the back-side feed port passes the reaction tube 51 substantially in parallel with the substrate disposed therein to be discharged from the front-side exhaust port, as indicated by an arrow in FIG. 7(b). At this time, the valves 61, 64 are opened, whereas the valves 62, 63 are closed.
Subsequently, as shown in FIG. 7(c), an oxygen gas is supplied to the reaction tube 51. The flow of the oxygen gas thus supplied is the same as that of FIG. 7(b) referred to above. After the supply of the oxygen gas, the substrate heating step is also finished, and the control process proceeds to the following film forming step.
FIGS. 8(a) through 8(e) illustrate the states of ventilation or gas flows in the reaction tube during the film forming step.
In FIG. 8(a), a gas flow through the apparatus in a first stage of the film forming step is shown. A film-forming gas comprising oxygen and evaporated penta-ethoxy-tantalum is supplied, together with a carrier gas in the form of a nitrogen gas, to the heated reaction tube 51 and thermally decomposed there to form a tantalum oxide film on the substrate (not shown). At this time, the flow of the film-forming gas is the same as that of FIG. 7(b), but with the valve 61 being fully opened.
Subsequently, as shown in FIG. 8(b), the valves 61 through 64 are all opened so that a film-forming gas flows from the back-side feed port to the back-side exhaust port, and another film-forming gas flows from the front-side feed port to the front-side exhaust port. Such valve opening operations are carried out in order to allow, in a second stage of the film forming step following the first stage thereof, a fresh film-forming gas to flow in a direction opposite that in the first stage.
FIG. 8(c) shows the flow of a gas through the apparatus in the second stage of the film forming step. In this figure, a film-forming gas together with a carrier gas in the form of a nitrogen gas is supplied to the heated reaction tube 51 and thermally decomposed there to form a tantalum oxide film on the unillustrated substrate. In the second stage of the film forming step, the film-forming gas supplied from the front-side feed port passes the interior of the reaction tube 51 substantially in parallel with the substrate therein to be exhausted from the back-side exhaust port, as indicated by arrows in FIG. 8(c), At this time, the valves 62, 63 are opened (in particular, valve 62 is fully opened), whereas the valves 61, 64 are closed.
After the film formation has finished, as shown in FIG. 8(d), the valves 61, 62 are closed and the valves 63, 64 are opened so that a residual gas in the reaction tube 51 is discharged by means of the discharge pump (DP) from the back-side exhaust port and the front-side exhaust port to the outside of the reaction tube 51 through the exhaust pipe 47.
Finally, as shown in FIG. 8(e), a nitrogen gas is supplied to the reaction tube 51, as in the stand-by state of FIG. 7(a), and the entire process is over. Though not shown, during a substrate taking-out step in which the substrate having the films thus formed is taken out of the reaction tube 51, all the valves 61, 62 for the gas feed ports are closed and the interior of the reaction tube 51 is discharged or vacuum drawn from the exhaust ports by means of the discharge pump (DP) so as to be at a desired pressure.
With the above-mentioned conventional film forming method, however, a problem arises in that when a tantalum oxide film is to be formed on a substrate for example, it is difficult to provide such a tantalum oxide film uniformly on the substrate. For example, in an attempt to form a tantalum oxide film on a substrate according to the aforesaid film-forming recipe by using the semiconductor processing apparatus as disclosed in the above-mentioned Japanese Patent Application Laid-Open No. Hei 7-94419, residual components of a film-forming gas which had adhered to the back-side exhaust port are liable to diffuse and flow back into the reaction chamber as a film-forming gas during the substrate heating step, thus resulting in the formation of a thick tantalum oxide film on a portion of the substrate near the back-side exhaust port.
Moreover, another problem is that upon removing the residual gas after the film forming step, as well as during the substrate introducing step and during the substrate taking-out step, the residual components of the film-forming gas attached to the gas exhaust ports are apt to diffuse and flow back into the reaction chamber as a film-forming gas, thus being deposited on the substrate and deteriorating the uniformity in the film thickness. This is because, even if the reaction chamber is discharged or exhausted through vacuum drawing with no gas being supplied thereto, it is difficult to prevent a reverse diffusion into the reaction chamber of the film-forming gas components remaining in the gas exhaust ports to any satisfactory manner.